Android - Broadcast Receivers

Broadcast Receivers can be considered as both Application Components and Inter Process Communication (IPC) mechanisms
As an IPC mechanism, Broadcast Receivers enable communication between different applications by sending and receiving Intents
These Intents can be sent by the Android system, other apps, or the app itself
As an Application Component, Broadcast Receivers are designed to respond to system-wide or custom events broadcasted by other applications
Broadcast Receivers can act as a messaging system between different components across the Android ecosystem
- For example, the system broadcasts an event when the device starts charging
- Similarly, an app can send a custom broadcast to let other apps know that new data has been downloaded
Broadcast Receivers extend the BroadcastReceiver class and override the onReceive() method to match a specified Intent Filter declared in the AndroidManifest.xml
The following example shows a Broadcast Receiver handling an event where the device is charging.

public class MyBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

        if (action != null) {
            switch (action) {
                case Intent.ACTION_POWER_CONNECTED:
                    // Handle the power connected event
                    break;
                case Intent.ACTION_POWER_DISCONNECTED:
                    // Handle the power disconnected event
                    break;
                default:
                    // Handle other actions as needed
                    break;
            }
        }
    }
}

Broadcast Receivers also need to be declared in the AndroidManifest.xml file.

<manifest ...>
    <application ...>
        <receiver android:name=".MyBroadcastReceiver">
            <intent-filter>
                <action android:name="android.intent.action.ACTION_POWER_CONNECTED" />
                <action android:name="android.intent.action.ACTION_POWER_DISCONNECTED" />
            </intent-filter>
        </receiver>
    </application>
</manifest>

The following methods are used for sending broadcasts to different kinds of receivers.

Method	Description
`sendOrderedBroadcast(Intent, String)`	Sends broadcasts to one receiver at a time.
`sendBroadcast(Intent)`	Sends broadcasts to all receivers in an undefined order.
`localBroadcastManager.sendBroadcast(intent)`	Send Intent broadcasts to local objects within your process .This method is deprecated since API 28, and `LiveData` is used instead.

Similar to Android Activity, broadcasting messages through ADB is also possible
Beginning with Android 8.0 (API level 26), the system imposes additional restrictions on manifest-declared receivers
Manifest can not be used to declare receivers for most implicit broadcasts (broadcasts that don't target your app specifically)
- However, exceptions exist and can be found here.